This invention relates to a system for controlling a rolling mill, and more particularly to a system for controlling the thickness of a strip being rolled by the rolling mill.
FIG. 1 shows one conventional control system for controlling the thickness of a strip being rolled by a rolling mill, together with the arrangement of rolling stands. In FIG. 1, rolling stands 11-15 (rolling stand 15 is hereinafter referred to as the final rolling stand) are arranged in tandem configuration. A strip is being rolled in the direction of the arrow x. The work-rolls of each of rolling stands 11-14 are bright rolls with smooth surfaces, but that of the final rolling stand is a matt roll with high surface roughness. Rolling with this sort of arrangement of rolls is called matt rolling.
In the abovementioned conventional control system for controlling the strip thickness, be it directly or indirectly, the deviation in thickness of the strip 1 on the output side of each of rolling stands 11, 12, 13 and 15 vis-a-vis the desired thickness is determined, and the roll gap or roll speed is controlled in such a manner that this deviation approximates to zero. However, there is no feedback control with regard to deviation in the thickness of the strip on the output side of Polling stand 14, i.e. on the input side of the final rolling stand 15.
For this reason, it is impossible to control any variation in strip thickness which occurs on the input side of the final rolling stand 15 until variation in the strip thickness on the output side of the final rolling stand 15 occurring as a result of this variation (variation in finished strip thickness) is detected by the thickness gauge 65, and this results in variation in the finished strip thickness. Moreover, since the variation in the finished strip thickness is controlled by adjusting the rolling speed of the antepenultimate rolling stand 13, which is located more upstream than the stand 14 adjacent to the stand 15, speedy control response to the variation is not feasible.
When the work rolls of the rolling stands in tandem configuration are all bright rolls, the tension of the strip between the final rolling stand and the penultimate rolling stand is maintained constant by feeding the amount by which the roll gap is to be corrected to a hydraulic device for controlling a roll gap of the final rolling stand. In matt rolling, however, where the work roll of the final rolling stand is a matt roll, it is necessary to maintain the rolling load and rolling speed of the final rolling stand at an initially predetermined value in accordance with the rolling schedule. Conventionally, the rolling speed of the penultimate rolling stand is corrected in such a manner that the detected tensile value on the input side of the final rolling stand accords with the desired tensile value.
Thus, in matt rolling, it is further necessary to correct the rolling speed of the antepenultimate rolling stand vis-a-vis correction of the deviation in strip thickness on the output side of the final rolling stand. As a result of this it is impossible to control the deviation in strip thickness on the input side of the final rolling stand because the amount by which the rolling speed is to be corrected is being fed to the motor drivers of all the rolling stands with the exception of the final one.